1. Field of the Invention
This invention relates to electrophotography, and more particularly to an improved method and apparatus for controlling the level of electrostatic charge on a surface upon which an electrophotographic image is to be made.
2. Description of the Prior Art
In known electrophotographic reproduction apparatus such as copiers or duplicators, an electrostatic charge is deposited on an area of a recording element, e.g., a photoconductor as the area is moved past a charging station. The photoconductor is then moved to an exposure station where the area is exposed to image-forming radiation to form a latent electrostatic image of a document to be copied. The latent image is thereafter developed and, in the case of plain-paper copiers and duplicators, subsequently transferred to paper upon which the copied image is to appear. Thereafter, the photoconductor is cleaned and otherwise made ready for the next copy cycle.
In such apparatus, it is important to impart a generally uniform charge over the area upon which the latent image is to be formed. Too low a charge in portions of the area may result in weak, washed-out looking areas on copies, and too great a charge in portions of the area may result in areas on copies being too dark relative to other areas. Therefore, copy quality, particularly with pictorial subject matter, is affected seriously where a non-uniform charge is placed on the photoconductor.
In the prior art, the known electrostatic charging devices with corona discharge can be classified into two categories, one using corona wire or line electrodes and the other using needle or point electrodes. The device using a corona wire electrode comprises a wire strung transverse to the direction of movement of the photoconductor. Generally speaking, this device is easier to manufacture but is known to have uneven discharge along the wire resulting in nonuniform electrostatic charging, particularly in the case of negative corona discharge. Discrete glow spots often occur along a negatively charged corona wire. The glow spots are associated with creating non-uniformities in charging of the photoconductor. As the glow spots appear at different positions along the wire over the course of a day due to changes in humidity within the copier or because of other factors, the non-uniformity of charging will thus change with time (see R. M. Schaffert, Electrophotography, 1975 edition, pages 466-472). While minor amounts of non-uniformity may be tolerated, significant non-uniformity as indicated above presents a problem. The extent of the problem will depend upon the nature of the apparatus and the material to be copied. Obviously, continuous tone or halftone originals will be more of a problem than copying text. Color copiers demand even more uniformity in charging than do monotone copiers.
Devices using needle or point electrodes arranged in a row across the photoconductor are known to provide an electrostatic potential distribution which is not uniform but which show a repeated pattern corresponding to the arrangement of the electrodes. The pattern appears in streaks along the direction of movement of the surface to be charged with respect to the arrangement of the needle electrodes. As noted in U.S. Pat. No. 3,649,830, various attempts have been undertaken to increase the uniformity of charging using needle electrodes such as by reducing the angle between the row of needles and the direction of movement of the photoconductor or causing the electrodes to be reciprocated; i.e., moved back and forth across the photoconductor, to minimize the pattern effect. The former attempt requires a larger space for a charging device than in the example where the needles are arranged in a row perpendicular to the direction of movement of the photoconductor whereas the latter attempt requires a relatively complex apparatus for reciprocating the needles.
It would therefore be very desirable to have an improved method and apparatus for providing over the image-forming area of a photoconductor a generally uniform charge.